Semi-automatic firearms, such as rifles and shotguns, are designed to fire a round of ammunition, such as a cartridge or shot shell, in response to each squeeze of the trigger of the firearm, and thereafter automatically load the next shell or cartridge from the firearm magazine into the chamber of the firearm. During firing, the primer of the round of ammunition ignites the propellant inside the round, producing an expanding column of high pressure gases within the chamber and barrel of the firearm. The force of this expanding gas propels the bullet/shot of the cartridge or shell down the barrel.
In semi-automatic rifles and shotguns, a portion of the expanding gases typically are directed through a duct or port that interconnects the barrel of the firearm to a piston assembly that generally houses an axially moveable gas piston. This piston assembly further typically includes a gas block that connects the piston assembly to the barrel, and through which the explosive gases pass. In some systems, the gas blocks are one piece elements located on their firearms and aligned with the port in the barrel through which the gases from the fired cartridge flow into the gas block and back to the action for expelling the spent cartridge and for chambering a fresh cartridge. The portion of the explosive gases that are diverted from the barrel of the firearm act upon the gas piston so as to force the gas piston in a rearward direction to cause the rearward motion of the bolt of the firearm. This rearward motion of the bolt opens the chamber, ejects the empty shell or cartridge casing, and thereafter loads another shell or cartridge into the chamber, after which the bolt returns to a locked position for firing as the gases dissipate or are bled off.